A headrest position adjustment device (for example, see Patent Document 1 (pp. 6-8, FIG. 1 to FIG. 6)) is known as a technique for adjusting the position of a headrest provided so as to correspond to a seat of a vehicle such as an automobile, etc. This headrest position adjustment device includes a capacitance sensor unit and a drive motor unit. The capacitance sensor unit includes a plurality of sensing electrodes and a detecting circuit.
The detecting circuit calculates an estimated center position of a head in the height direction based on sensing signals from the respective sensing electrodes, so that the position of the headrest with respect to the seat is adjusted based on the estimated center position.